A central effort in the field of molecular electronics has been the investigation of molecules as rectifiers, switches, storage devices, etc., in microelectronic applications. The goal of the present invention is to provide a means to alter junction conductivity via a chemical change induced by an electrical stimulus.
Electron transport (ET) through molecules in molecular junctions is fundamental to the area of molecular electronics, and understanding the relationship between molecular structure and transport is a prerequisite to the eventual design of molecular circuit components. The factors which determine ET through molecules oriented between metallic contacts has been investigated in single molecule devices based on scanning probe microscopy, as well as molecular junctions containing 103-1012 molecules in parallel. It is clear from the results reported to date that ET depends on molecular structure as well as the nature of the “contacts” between the molecules and the conductors. Several reviews on molecular junctions and molecular rectification have appeared, and research on ET mechanisms in such devices remains active.
Several laboratories have investigated “hybrid” devices involving the interface between a molecule and a conventional semiconductor such as silicon, gallium arsenide, and TiO2. Research areas as diverse as dye sensitized solar cells and molecularly modified quantum dots also involve a combination of molecular and semiconductor properties. Our lab has investigated carbon/molecular/TiO2/Au junctions in some detail, and compared them to similar junctions lacking the TiO2 layer. By combining a molecular and semiconductor layer in the junction, one can in principle exploit the distinct electronic properties of each material in a “molecular heterojunction”. For example, robust negative differential resistance has been observed in polyphenylene vinylene/TiO2 heterojunctions, with the phenomenon being attributed to alignment of energy levels between the PPV and TiO2. We have described carbon/nitroazobenzene (NAB)/TiO2/Au junctions in which electrons are transferred between the NAB and TiO2 to produce rectification and conductance switching.
During both the spectroscopic and electronic characterization of Carbon/NAB/TiO2/Au junctions, both transient and persistent conductance changes were observed, whose origin was not completely clear. Potential pulses applied to such junctions produced a transient current lasting a few msec, followed in some cases by a slow (10-100 msec) conductance increase which persisted for several minutes. This “memory” effect may have technological value as well as mechanistic consequences, and deserved further study. Although Raman Spectroscopy established that NAB is reversibly reduced and oxidized in NAB/TiO2 junctions only ˜8 nm thick, the fate of the TiO2 accompanying NAB redox events was not amenable to Raman monitoring, and was not determined directly. It is clear from these investigations that electron injection into the TiO2 film is directly involved in the electronic behavior of the junction, but the events accompanying injection are not yet evident, including their relationship to observed changes in junction conductance.
The present invention clearly defines the conductance changes which occur when carbon/molecule/TiO2/Au heterojunctions are subjected to an applied electric field. Fluorene (FL) junctions were investigated rather than previously studied NAB devices in order to simplify the problem by reducing the possibility of redox reactions in the molecular layer. Carbon/FL/TiO2/Au junctions were compared to carbon/TiO2/Au analogs to determine the influence of the FL layer on electronic behavior. In addition, a carbon/Al2O3/TiO2/Au junction was investigated as an analog with a high-barrier insulator in place of the FL layer.
In view of the present disclosure or through practice of the present invention, other advantages may become apparent.